fix for encoding error when loading custom map with unicode

error message `'btoa' failed: The string to be encoded contains characters
outside of the Latin1 range` when some sourcesContent contains unicode

applied fix detailed in this MDN article 
<https://developer.mozilla.org/en-US/docs/Web/API/Window.btoa>

Couldn't test that the linking still works since interacting with the
"link here" link crashes my browser (my source map is 1.4MB)